Method of fusing alloy additions to a steel bath



Patented June 19, 1951 METHOD OF FUSIN G ALLOY ADDITIONS T A STEEL BATH Albert C. Ogan, Pleasant Hills Borough, 'Pa., assignor to United States Steel Company, a corporationof New Jersey No Drawing. Application March 31, 1950, Serial No. 153,297

4 Claims.

This invention relates to the manufacture of alloy steel and, in particular, to a method of introducing alloying elements into a bath of molten low-carbon steel.

In the manufacture of high-alloy steel it is necessary, after producing a heat of steel of the desired carbon content, to add the required amounts of alloying elements. It has been proposed to add these elements in molten form but economic considerations dictate the present almost universal practice of making alloy additions in solid form. If the amounts of such additions be small, they are melted by the excess heat of the bath and their effect on the temperature of the bath is negligible. In the case of high-alloy steels such as stainless steel, however, the amounts of the additions are so large that it is necessary to apply heat externally for melting them in order to avoid undesiiably reducing the bath temperature. This may be done in an open-hearth furnace by the combustion of additional fuel and in an electric furnace by increasing the current supplied to the arc electrodes. dizing atmosphere which reduces the recovery or yield of the alloy additions. Increased electric current, while free from this objection, is subject to others under certain conditions.

A typical example of the undesirable effect of an electric arc is encountered in the making of stainless steels. These alloys frequently use onefifth of their weight of chromium and, furthermore, call for a low-carbon content. The alloy requirement is partially met by the use of an ex pensive low-carbon ferrochromium which is added to the properly decarburized and deoxidized bath. It has been found, however, that the high temperature and highly carburized atmosphere of the furnaces in which additions are made, cause an undesirable recarburization of chromium and, subsequently, of the steel. As long as pieces of the alloy are covered with the slag, the carburizing action is slight, but since economic operations dictate the charging of large amounts of additions at a time, much of the ferrochromium remains above the level of slag, being fully exposed to the carburizing action of both the electric arc and the atmosphere. In the presence of an electric arc, the carbon of the electrodes is ionized providing a continuous stream of active particles of the element which are absorbed by the bath. Because of the low voltage used, the electrodes are quite close to the surface of the Fuel combustion creates an oxibath, frequently being submerged in the slag. As

a result, they generate carbon monoxide which is readily absorbed by the metal. These two factors in combination lead to an increase of carbon concentration which is seriously objectionable.

I have invented a method of adding to a bath of molten steel, alloying elements used in large amounts, and supplying the heat needed to melt them without creating carburizing conditions above the bath. In a preferred practice, I add silicon and aluminum along with the alloy addition and supply oxygen to cause combustion of the two former. The products of this reaction cannot contribute to recarburizing the alloy addition or the steel bath and the effects of the oxidation on the bath or the slag floating thereon may be readily counteracted. In the case of an electric-arc furnace, the arcs are discontinued during the addition and fusion of the alloy.

A complete understanding of the invention may be obtained from the following detailed description of an example of the preferred practice.

The method of my invention can best be illustrated by describing the making of a heat of stainless steel in an electric-arc furnace by the aid of said method. The furnace used was of 45 tons capacity. It was charged with 13,370 pounds of stainless-steel scrap and 52,600 pounds of-armor-plate scrap which was melted and decarburized to a carbon content of about 25% in the usual manner by using an oxygen lance. Since large amounts of chromium oxide resulted from the oxygen treatment used for carbon reduction, this oxide was then reduced and incorporated into the metal along with added chromium by adding ferrosilicon and then aluminum to the slag, as explained in detail herebelow, after which the slag was removed from the furnace following the conventional slag-off practice.

In this stage, the current to the furnace was shut off, the electrodes raised above the roof, and the addition of lump ferrochromium was started. Altogether 21,270 pounds of ferrochromium (.06% carbon) was added in live batches of substantially the same weight (about two tons). The charging of each addition consumed about five minutes and the additions were completed thirtyfive minutes before tapping to enable a proper shaping of the slag. Fresh lime, together with pounds of powdered ferro silicon and 60 pounds of aluminum, was spread over the bath after each ferrochromium addition and oxygen was applied to the slag through a conventional oxygen lance by pointing the lance towards the bank at an angle of substantially 60 in such manner as to cause a swirling action of the slag and to cause the fusion of slag-forming materials within five to ten minutes.

The batch of two tons of ferrochromium delivered by the charging box at each addition formed a solid island floating on the molten bath. The oxygen was supplied under a pressure of 100 .p. s. i. through a steel pipe inserted under the slag in close proximity to the edges of the ferroohromium pile. While only a comparatively slight generation of gases was noted, an intense heating was produced by the action of oxygen on the silicon and aluminum in the slag in the neighborhood of the ferrochromium, and a rapid melting thereof ensued. Each of the several batches of ferrochromium was charged rapidly following the melting of the previous batch. In spite of the cooling effect of the ferrochromium additions and the absence of the heating arcs, the temperature of the bath did not appear to be lowered during the hour and a half required for charging and melting the ferrochromium. At the end of this period, the electrodes were lowered, the power was turned on re-establishing the arcs, and the slag was shaped in the usual manner, requiring thirty-five minutes for the operation.

The discontinuance of the arcs and the use of an oxygen lance for increasing the rate of melting of the ferrochromium resulted in reduced current consumption and in substantial prevention of the recarburization of the alloy addition and of the bath itself. Using ferrochromium containing 0.06% carbon, final furnace tests showed the steel had carbon contents of 0.04% and 0.05% while ladle tests further proved the absence of recarburization, showing 0.05% carbon.

The application of oxygen for melting ferrochromium in an electric furnace by the use of the oxygen lance, with a slag carrying silicon and aluminum, apparently does not increase the normal oxidation rate or cause excessive losses of chromium. Tests conducted for determining the losses attributable to the use of oxygen showed a recovery of 98% of the ferrochromium added as the alloy, while the total recovery of chromium (both that in the original charge and that added as ferrochromium) was 93%, both figures indicating a normal performance.

The foregoing specific illustration of my improved method applied to alloying steel with chromium is not to be considered as limiting the process to chromium additions. Manganese, for example, used in amounts required for austenitization of steels of the Hadfield type can be incorporated into the molten bath held in a furnace by following the teachings of the invention.

In the latter case, there is little or no concern as to the contamination of the alloy with carbon, but the use of the method is amply justified in the light of time and current savings.

The specific character of high-alloy steels requiring large additions of non-ferrous elements indicates that an electric furnace is a particularly suitable device for their manufacture, utilizing the present invention. It is to be understood, however, that th method may be practiced in other metallurgical furnaces, such as open-heart furnaces, when individual conditions warrant its application.

I claim:

1. In the manufacture of alloy steels in an electric arc furnace, the steps of adding solid alloys to the layer of slag floating on a molten bath of steel, discontinuing the furnace arcs and introducing a stream of oxygen into the slag layer adjacent said alloys to fuse them.

2. In a method of making alloy steel, the steps including making a heat of low-carbon steel, removing the slag therefrom, at least partially, replacing the slag with lime having powdered aluminum and silicon mixed therewith, adding lump ferrochromium to the bath and then supplying oxygen to the bath adjacent the ferrochromium whereby the resulting oxidation of the silicon and aluminum generates heat to aid the fusion of the ferrochromium.

3. The method defined by claim 2 characterized by making said heat in an electric-arc furnace and discontinuing the furnace arcs during charging and fusion of the ferrochromium additions.

4. In the manufacture of alloy steels, the step of making a heat of low-carbon steel under a layer of molten slag, adding silicon and aluminum to the slag, then adding ferrochromium to the slag and introducing a stream of oxygen under the slag at a point adjacent to said lastmentioned addition thereby fusing the latter by the heat evolved by the resulting oxidation of the silicon and chromium.

ALBERT C. OGAN.

REFERENCES CITED The following references are of record in the file of this patent:

Steel, Dec. 13, 1948, page 104. Published by the Penton Publishing Company, Cleveland, Ohio. 

1. IN THE MANUFACTURE OF ALLOY STEELS IN AN ELECTRIC ARC FURNACE, THE STEPS OF ADDING SOLID ALLOYS TO THE LAYER OF SLAG FLOATING ON A MOLTEN BATH OF STEEL, DISCONTINUING THE FURNACE ARCS AND INTRODUCING A STREAM OF OXYGEN INTO THE SLAG LAYER ADJACENT SAID ALLOYS TO FUSE THEM. 